In an effort to overcome the volatility of dynamic random access memory (hereinafter referred to as DRAM), which is a semiconductor memory device that is widely used in electronic devices such as personal computers (PCs) and mobile phones, studies on magnetic random access memory (hereinafter referred to as MRAM) having non-volatile memory characteristics have been actively conducted. As used herein, the term “non-volatile memory” refers to the property that requires a specific amount of power only to read and write information and that keeps written information without requiring separate power even when a power supply is blocked. Particularly, in recent years, the density of integration of the DRAM reached a limit, and thus the MRAM has been considered as a substitute for the DRAM. Therefore, in the related industrial fields, the research and development of the MRAM has been actively conducted.
Studies on the MRAM have been conducted since the early 2000s, and early studies were focused mainly on changing the resistance of tunneling magneto-resistance (hereinafter referred to as TMR) devices by reversing magnetization using a magnetic field created by application of an electric current. However, this TMR-based MRAM device has a shortcoming in that, as the size of the device decreases, the amount of writing current greatly increases, making it difficult to realize large-scale, densely integrated memory. Due to this shortcoming, an MRAM technology based on spin-transfer torque magnetization switching was introduced. It is a type of current-induced magnetization switching, and is based on a method of switching magnetization using a spin-transfer torque (hereinafter referred to as STT) generated by applying a current to a magnetic thin film. The MRAM based on this method is referred to as STT-MRAM. Spin-transfer torque magnetization switching provides various advantages, including high integration density, wide write window and low power consumption, compared to existing magnetic field-induced magnetization switching.
Prior studies on the STT-MRAM were focused mainly on magnetic tunnel junctions (hereinafter referred to as MTJs) with in-plane magnetic anisotropy. Recently, in-plane magnetic tunnel junctions (iMTJs), which have a relatively low critical current density while maintaining their thermal stability in nanosized magnetic cells, were also developed. Such results were mostly obtained in MgO-based structures having an exchange-coupled trilayer including a free layer and a pinned layer, but a MTJ that requires a lower critical current density (e.g., 1 MA/cm2 or less) is required to realize a highly integrated MRAM device for commercial use.
In view of this disadvantage of iMTJ, an MJG with perpendicular magnetic anisotropy (hereinafter referred to as PMA) has a very big advantage in that the critical current density required for magnetization switching is low. This is because the iMTJ requires additional torque to overcome a demagnetizing field (2n Ms, where Ms=saturation magnetization) during magnetization switching, and thus it is difficult to lower the critical current density. For this perpendicular MTJ (pMTJ), it is most important to develop materials and structures, which have excellent PMA properties (PMA energy density=about 107 erg/cc). However, from the view point of magnetostatic energy, PMA should overcome a very high demagnetizing field, and thus it is fundamentally difficult to develop materials and structures, which have excellent PMA properties.
PMA can be largely divided into PMA caused by interfaces, and PMA caused by bulk properties. Until now, three kinds of materials with perpendicular magnetic anisotropy (PMA) have been mainly studied, including rare earth-3d transition metal amorphous alloys, multilayer thin films such as CoPd and CoPt [W. B. Zeper et al., J. Appl. Phys. 70, 2264 (1991)], and intermetallic compounds, such as FePt, CoPt, which have the L10 structure [T. Shima et al., Appl. Phys. Lett. 80, 288 (2002)].
However, rare earth-3d transition metal amorphous alloys have problems in that the PMA energy density is insufficient and in that crystallization occurs even at a relatively low temperature (about 300° C.) to rapidly reduce the PMA properties. On the other hand, intermetallic compounds such as FePt and CoPt, which have the L10 structure, are known as materials having the best characteristics up to date, because the PMA energy density is sufficiently high and the temperature characteristics are also good. However, the intermetallic compounds with the L10 structure also have a problem in that these compounds are not suitable for temperature conditions that are used in current memory device processes, because a temperature higher than 600° C. is required to form an intermetallic compound having a high long-range order known as the most important factor for PMA. In addition, there is a problem in that it is not easy to design a seed layer and a buffer layer, which are required to form the (001) texture essential for perpendicular magnetic anisotropy (PMA). Finally, multilayer thin-film structures such as CoPd and CoPt have sufficient. PMA energy density, but have a problem in that these multilayer thin-film structures are easily broken down at a temperature ranging from about 350° C. to 450° C., which is the heat treatment temperature used in current memory fabrication processes, and thus the PMA properties are reduced or lost.
Accordingly, due to the above-described problems occurring in the art, there is an urgent need for a new material and structure, which are suitable for the heat treatment temperature that is used in current memory fabrication processes, and at the same time, has sufficient perpendicular magnetic anisotropy density.